A known analog electronic circuit includes a power supply circuit for supplying power to each circuit. In order to activate the power supply circuit, a predetermined amount of electric charge is stored in a capacitor and then supplied as a starting current to the power supply circuit, which leads to an activation of each circuit. WO02/007302A1 discloses a technique for stably activating a piezoelectric oscillator by applying a predetermined level of voltage for promoting the start to a piezoelectric vibrator for the time required, by means of an instantaneous voltage supply means consisting of a capacitor and a transistor.
Recently, an electric circuit or a system that detects an approach or an existence of a human and then automatically starts operation has been developed. According to such the electric circuit or the system, a power supply other than a sensor circuit is turned to OFF position (i.e. in sleep mode) until the approach or the existence of the human is detected. That is, the power supply other than the sensor circuit is started when the approach or the existence of the human is detected so as to reduce as much standby power consumption as possible.
Precisely, for example, a system that has been increasingly used in a vehicle door or a front door of a house detects a change in capacitance in the vicinity of a door handle or a doorknob as soon as the approach of a user carrying a key with an ID code is detected, and then automatically unlocks the door. According to this system, which is called a smart entry system, a user carrying the key can unlock the door by approaching the door and touching the door handle or the doorknob by her/his hand as the change in capacitance is detected, The door in a locked state can be unlocked and opened by the user simply pulling the door handle or the doorknob accordingly.
Meanwhile, according to the aforementioned system, a circuit for determining whether the door should be unlocked or not performs a rather simple sequence such as detecting the change in capacitance in the vicinity of the door handle or the doorknob, and outputs a signal for unlocking the door. Therefore, a small digital circuit is frequently used for such the circuit. Further, such the circuit has been practically used in the smart entry system for a vehicle and thus the circuit needs to start as soon as the user carrying the ID key approaches to the door, detect the change in capacitance in the vicinity of the door handle, determine whether or not his/her hand touches the door handle, and then automatically unlocks the door. This function is necessary since at a time of the user carrying the ID key running to approach the door under a rainy weather and the like and operates the door handle, the smart entry system needs to function instantaneously so as to unlock the door.
Since the instantaneous voltage supply means disclosed in WO02/007302A1 uses a capacitor, the charging time is required for activation of the power supply circuit if the capacitor lacks the electric charge, and at the same time the power is consumed in the capacitor. As a result, the use of such the starter circuit in the smart entry system may cause increase of the starting time and the power consumption.
Thus, a need exists for a starter circuit that can be instantaneously started with reduced power consumption.